You might want to check for all mapped drives first. If a drive letter is already mapped, it will fail. I got the below code from the UT along time ago:

* Constants for drive types.
#DEFINE DRIVE_UNKNOWN 0
#DEFINE DRIVE_NO_ROOT_DIR 1
#DEFINE DRIVE_REMOVABLE 2
#DEFINE DRIVE_FIXED 3
#DEFINE DRIVE_REMOTE 4
#DEFINE DRIVE_CDROM 5
#DEFINE DRIVE_RAMDISK 6
#DEFINE CR CHR(13)

DECLARE INTEGER GetLogicalDrives IN Win32API
DECLARE INTEGER GetDriveType IN Win32API STRING RootPath

PRIVATE lcDrivestring, lcDriveRoot, liDrivelist, lcDrives, i
lcDrives = ""

* GetLogicalDrives returns a 32-bit value containing a list of
* available drives. If the specific bit is ON, the drive letter
* corresponding to that position exists.
liDrivelist = GetLogicalDrives()

* Check each of the bits
FOR i = 0 TO 31
	IF BITTEST(liDrivelist, i)
* If the 0th bit is ON, that means drive 'A:',
* the 2nd bit is 'C:", etc.
		lcDriveRoot = CHR(65 + i) + ":\"
* Perform a GetDriveType to determine if it is a floppy, CD, etc.
		lcDrivetype = GetDriveType(lcDriveRoot)
		DO CASE
			CASE lcDrivetype = DRIVE_UNKNOWN
				lcDrivestring = "Cannot be determined"
			CASE lcDrivetype = DRIVE_NO_ROOT_DIR
				lcDrivestring = "Root directory does not exist"
			CASE lcDrivetype = DRIVE_REMOVABLE
				lcDrivestring = "Floppy/removable drive"
			CASE lcDrivetype = DRIVE_FIXED
				lcDrivestring = "Hard drive/nonremovable drive"
			CASE lcDrivetype = DRIVE_REMOTE
				lcDrivestring = "Remote/Network drive"
				lcunc=GetUNCForDriveLetter(LEFT(lcDriveRoot,2))
				IF TYPE('lcunc')="C" .AND. !EMPTY(lcunc)
					lcDrivestring = lcunc
				ENDIF
			CASE lcDrivetype = DRIVE_CDROM
				lcDrivestring = "CD-ROM drive"
			CASE lcDrivetype = DRIVE_RAMDISK
				lcDrivestring = "RAM disk"
		ENDCASE
		lcDrives = lcDrives + lcDriveRoot + " " + lcDrivestring + CR
	ENDIF
NEXT

=MESSAGEBOX(lcDrives, ;
	0 + 64 + 0,'All Drives Listing')
RETURN


FUNCTION GetUNCForDriveLetter
LPARAMETER cDriveLetterToCheck
LOCAL cBuffer, nResult, lcunc
IF TYPE('cDriveLetterToCheck') # 'C' OR ;
		LEN(cDriveLetterToCheck) < 2 OR ;
		SUBSTR(cDriveLetterToCheck,2,1) # ':' OR ;
		! BETWEEN(UPPER(LEFT(cDriveLetterToCheck,1)),'A','Z')
	RETURN ''
ENDIF
DECLARE INTEGER WNetGetConnection IN Win32API ;
	STRING   @cLocalDrive, ;
	STRING   @cRemoteUNCBuffer, ;
	INTEGER  @nSizeOfBuffer
cBuffer = SPACE(511)
nResult = WNetGetConnection(LEFT(cDriveLetterToCheck,2), ;
	@cBuffer, ;
	511)
IF nResult # 0
	RETURN ''
ELSE
	lcunc=LEFT(cBuffer,AT(CHR(0),cBuffer)-1)
	RETURN lcunc
*RETURN LEFT(cBuffer,AT(CHR(0),cBuffer)-1)
ENDIF

>>>>I think WinXP can open ZIP-ed files. Also why you dind't make backup as a Selfextract file? The you didn't need bat, PKUNZIP etc. files.
>>>
>>>Yes, you are right, WinXP will open my ZIP files. But often times the computer is Win2K where this feature is not available. I could do the Selfextracting file, I have not thought about it. Thank you very much.
>>>
>>>But I am still wondering if there are other ways to open my .ZIP file, besides the Selfextracting approach.
>>
>>Don't know, maybe something like temporary maping the drive and then release it in BAT file:
>>NET USE Z: \\UNCpathHere
>>pkunzip z:\zip file
>>NET USE Z: /DELETE
>
>I will try this approach. I know I tried to map the drive from the Windows Explorer and had a problem (maybe it was looking for a password). But maybe from another letter-mapped drive it will work.
>
>Thank you very much for your suggestion.